Nickel-chromium-iron alloys are used primarily for their oxidation resistance and strength at elevated temperatures. Such alloys may be used, for example, as sheathing for electric heating elements and thermocouples.
Electrical resistance alloys are found in instruments and control equipment to measure and regulate electrical characteristics and in furnaces and appliances to generate heat. In the latter applications, elevated temperature characteristics are of prime importance. In common commercial terminology, electrical resistance alloys used for generation of heat are referred to as resistance heating alloys.
Resistance heating alloys such as CHROMEL-A.RTM.(80 Ni; 20 Cr) are used in many varied applications--from small household appliances to large industrial furnaces. In appliances, resistance heating elements are designed for intermittent, short-term service at about 100 to 1090.degree. C. (about 212 to 2000.degree. F.). In industrial furnaces, elements must often operate continuously at temperatures as high as 1300.degree. C. (2350.degree. F.); as high as 1700.degree. C. (3100.degree. F.) for kilns used for firing ceramics; and occasionally as high as 2000.degree. C. (3600.degree. F.) for special applications.
The primary requirements of materials used for heating elements are high melting point, high electrical resistivity, reproducible temperature coefficient of resistance, good oxidation resistance in furnace environments, absence of volatile components, and resistance to contamination. Other desirable properties are elevated temperature creep strength, high emissivity, low thermal expansion and lower modulus (both of which help minimize thermal fatigue), resistance to thermal shock, and strength and ductility or workability at fabrication temperatures.
It is known that nickel-chromium-iron compositions are ductile alloys and thus are workable. They play an important role in heaters for the higher temperature ranges. Such heaters are constructed to provide more effective mechanical support for the heating element.
Oxidation resistance of nickel-chromium alloys at elevated temperatures is weakened by the limited adherence of an initial protective oxide layer to the base metal. The ability of an alloy to survive long exposures of 2000.degree. F. and above in air depends in large part on this protective layer remaining intact. Alloys presently used for electrical heating elements are made with carefully controlled techniques of melting, refinement and fabrication to maximize performance. However, variations in chemistry that occur between melts lead to differences in performance, as measured, for example, by accelerated life tests.
Various theories have been suggested to explain why some melts may show substantial improvement over the average performance. However, there has not been positive confirmation of any theory that has led to predictable improvement in life test results and consistently reproducible performance.
Illustrative of certain nickel-based chrome-iron-aluminum alloys are those disclosed in JP 59-85836. Such alloys, however, have a VHN above 500, which is antithetical to workability.
U.S. Pat. No. 2,515,185 relates to nickel alloys, and more particularly to age hardenable nickel alloys. Such alloys, however, do not exhibit the requisite ductility. Nor does this patent envision the use of elements which promotes oxidation resistance of the nickel alloys. In addition, the precipitation of particles of secondary phases that promote hardening will adversely affect the resistivity of the base alloy, making it less desirable as an electrical heating element.
U.S. Pat. No. 4,460,542, which is incorporated herein by reference, calls for the addition of yttrium to a chromium-aluminum-iron alloy which exhibits resistance to oxidation at high temperatures. However, this alloy only requires 14-18% chromium. U.S. Pat. No. 4,671,931 abandons the concept of yttrium additions.